Many beneficial devices or structures in myriad applications are characterized at least in part by having a liquid that is in contact with at least one solid surface. Recent applications have focused on the movement of small droplets of liquid disposed on nanostructured or microstructured surfaces which can be manufactured by various methods, such as various means of lithography or etching. Such surfaces result in surfaces that are useful for significantly reducing flow resistance experienced by droplets of liquid disposed on the surfaces.
One such application is described in “Nanostructured Surfaces for Dramatic Reduction of Flow Resistance in Droplet-based Microfluidics”, J. Kim and C. J. Kim, IEEE Conf. MEMS, Las Vegas, Nev., January 2002, pp. 479-482, which is hereby incorporated by reference herein in its entirety. That reference generally describes how, by using surfaces with predetermined nanostructure features, the flow resistance to the liquid in contact with the surface can be greatly reduced. The Kim reference teaches that, by finely patterning the surface in contact with the liquid, and using the aforementioned principle of liquid surface tension, it is possible to greatly reduce the area of contact between the surface and the liquid. It follows that the flow resistance to the liquid on the surface is correspondingly reduced. However, as exemplarily taught by the Kim reference, the flow resistance to the liquid is reduced to such a level that it was difficult or impossible to control the movement of the liquid. Thus, it was necessary to dispose the droplets in a narrow channel or other enclosure to control the freedom movement of the droplet to within a prescribed area.
In order to better control the movement of liquid droplets disposed on surfaces patterned with nanostructures or microstructures, more recent attempts have relied on characteristics of the droplet or, alternatively, intra-pattern characteristics of the nanostructures or the microstructures to control the lateral movement of liquid droplets. Such control is the subject of copending U.S. patent application Ser. No. 10/403,159, filed Mar. 31, 2003, entitled “Method And Apparatus For Variably Controlling The Movement Of A Liquid On A Nanostructured Surface” which is hereby incorporated by reference herein in its entirety. In one embodiment described in that application, the lateral movement of a liquid droplet is achieved by designing, illustratively, the size, shape, density, or electrical properties of the nanostructure or microstructure such that the contact angle of the leading edge of a droplet is made to be lower than the contact angle of the trailing edge of the droplet. The resulting force imbalance causes the droplet to move in the direction of the leading edge. In another embodiment, the droplet is caused to penetrate the feature pattern at a desired area such that it becomes substantially immobile. This penetration can be affected, for example, by changing the surface tension of the droplet, the temperature of either the droplet or the pattern or the voltage differential between the droplet and the feature pattern.
As described in the '159 application, one or both of the above embodiments may be useful in a variety of applications, such as, illustratively, a biological or micro-chemical detector, a chemical reactor, a patterning application, a tunable diffraction grating, a total internal reflection mirror, a microfluidic mixer, a microfluidic pump or a heat dissipation device.
Thus, the above-described prior efforts focused on either reducing flow resistance experienced by a droplet or controlling the movement of a droplet of water across a surface. In another recent attempt, nanostructures or microstructures are used to reduce the flow resistance experienced by a body moving through a fluid. That attempt is described in copending U.S. patent application Ser. No. 10/649,285, entitled “Method And Apparatus For Reducing Friction Between A Fluid And A Body,” filed Aug. 27, 2003 and is hereby incorporated by reference herein in its entirety. According to the embodiments of the invention disclosed in the '285 application, at least a portion of the surface of a vehicle moving through a fluid is patterned with nanostructures or microstructures. Thus, according to the principles discussed above, the flow resistance across the patterned surface is reduced. Also as discussed above, by causing the fluid to penetrate the patterned surface, flow resistance across the patterned surface can be increased.